Electrophotography has become one of the most widely used systems in the field of information processing in this century. In particular, dry copying, also known as xerography, has become a standard process for creating copies of documents in a host of environments including offices, educational institutions and the like. The basics of xerography are well known to those skilled in the art.
The fundamental elements of a xerographic copier include a photosensitive medium which is charged with an electrostatic charge of predetermined polarity. An optical image of an original to be copied is focused onto the electrostatic medium, either at one time through the use of a stroboscopic flash and appropriate optics, or by a linear scan which moves either the light source, optical elements in the optical path, or both, in synchronism to scan the photosensitive medium with the image of the original.
Portions of the originally uniform electrostatic charge on the surface of the photoreceptor are conducted away from the areas of the photoreceptor which were illuminated, and thus an electrostatic image of the original is left on the photoreceptor. In most modern xerographic copying systems, this image is passed over a source of toner materials electrostatically held to ferromagnetic carriers. The ferromagnetic carriers are used to allow magnetic fields to bring the materials into contact with the above-mentioned electrostatic image.
The electrostatic charge which remains on portions of the electrostatic image has a sufficiently strong electrostatic force to pull the toner materials away from the carriers and to hold them in place on the appropriate portions of the electrostatic image. The magnetic forces associated with the toner modules carry the ferromagnetic carrier particles back to a position where they are remixed with additional toner.
As is known to those skilled in the art, the toner materials are normally plastics which melts at a predetermined temperature and have appropriate color characteristics once they are melted.
The charged photoreceptor which now carries toner on the portion of the photoreceptor which was not discharged in response to light when the electrostatic image was originally created, is referred to herein as a developed image. Subsequently, the photoreceptor carrying the developed image is brought into contact with an image receptor which, in the most common applications of xerography, is a sheet of paper. Electrostatic charging techniques are used to transfer the toner from the photoreceptor to the image receptor.
Once this is accomplished, the image receptor is passed through a device, commonly referred to as a fuser, which is a station in the path for the image receptor at which the transferred toner is heated to fix the image onto the image receptor. By this process, a monochrome copy of the original image which was exposed onto the surface of the photoreceptor is made.
In more recent years, systems for color electrophotography have been created. In many respects, the process of color electrophotography is analogous to standard three-color printing processes used in the more conventional printing arts. Conventional three-color printing color component images, commonly referred to as color separations, are created by photographing the original through appropriate filters. Each of the separations is in turn made into a separate printing plate. During the printing process, each plate is inked with an appropriate color determined by the filter used in making the original separation. The printing press can be adjusted so that proper registration, aligning each separate color component image over the other is accomplished. Once the press is properly adjusted, multiple copies of the original color image may be faithfully reproduced.
Prior art color electrophotography machines have used a conceptually similar process. Most full color dry copiers use three process colors to recreate (within the color limits of available toner materials) any color in the spectrum. Three color component images are shot through three separate filters in a manner analogous to the creation of color separations in color printing. Each image is developed with a toner having the appropriate color characteristics, and each developed color component image is in turn transferred to the image receptor or paper, one on top of the other, to provide a composite image. The paper carrying the composite image is then passed through a fuser in a conventional manner.
It is known in the art of color xerography to include an intermediate transfer medium between the above-described photoreceptor, upon which each individual color component image is developed, and the ultimate image receptor or paper. In this specification, such an intermediate transfer medium is referred to as a transfer medium. In this type of device, a composite developed image is built up, one color component image at a time, until an overlaid composite color image, having portions of all three of the color component toners thereon, is created on the transfer medium. Once this is accomplished, the composite image on the transfer medium is transferred to the paper which then passes through the fuser in the normal fashion.
As noted above, color xerography is conceptually quite similar to conventional color printing. However, there is a significant difference in the economics of scale. Most importantly, color printing is rarely undertaken for small numbers of copies. In practical environments, the color printer normally has ample opportunity to make sure that elements of the press are properly aligned so that proper registration is obtained. In the absence of proper registration, the individual color component images are misaligned and the result is a fuzzy image, with edges of objects being outlined inappropriately with portions of one of the color components. In color xerography, there is no ability by the user (nor the time) to make precise adjustments, since often one or two copies are all that are being made at any one time.
Therefore, in prior art color photocopying machines, the mechanical elements carrying the photoreceptor medium, the intermediate transfer medium (if used) and the paper have had to be machined to extremely close tolerances in order to maintain proper registration. In the prior art, this has only been practical by using relatively large drums to carry the photoreceptor and critically machined, and therefore expensive, gearing arrangements by which the entire mechanism is driven from a common prime mover. Naturally, as these mechanical components age and the mechanical elements controlling registration suffer wear, registration, and therefore copy quality can, and does suffer significantly. Therefore, maintenance of critical mechanical alignments in prior art full color electrophotographic systems have been one of the principal factors in keeping the costs of such machines very high with respect to the cost of monochrome copiers.
Additionally, it is well known to those skilled in the art that light sources of considerable intensity are needed in full color electrophotographic printing since, during the shooting of each color component image, a significant fraction of light from a white light source is blocked by the separation filters interposed in the optical path between the original and the photoreceptor. This, together with the power requirements of the conventional fuser mechanism and the electric motors to operate the above-referenced large mechanical drums, created a state of the prior art in which no full color copier known to the inventors of the present invention has ever been designed which can operate reliably from a conventional fifteen or twenty ampere 120 volt branch circuit.
Also, because of the use of finely machined parts and high power requirements, the prior art has heretofore not produced a practical full color copier which is of a size substantially equivalent to conventional table top monochrome copiers.
As is also known to those skilled in the art, conventional full color photocopying machines will output copies at approximately 1/3 the rate of an equivalent monochrome machine since three separate images must be developed for each copy. Additionally, all prior art full color copiers known to the inventors of the present invention have copied monochrome (black and white) originals by use of the standard color copying process. This is known in the art as creating a copy that is "process black". This refers to the fact that in conventional full color copiers, three separate substantially identical images are created when the machine is copying a black and white original. Each of these images is developed with a separate color component toner and the composite image is fused. If the toner color characteristics are right, and registration within the machine is adjusted properly, the resultant copy will approximate the black and white original.
However, it is well known to those skilled in the art that process black from color copying machines does not produce as sharp an image as conventional black and white copies made on monochrome xerographic machines using a single color black toner. Additionally, because three separate images are shot, even to copy a black and white original, the copy output rate for conventional full color copies has been too slow to encourage the user to make use of such a machine as a general purpose office copier.
The combination of the slower copying rate (due to creation of three separate images) and the lower copy quality of process black monochrome copies has made prior art full color copiers impractical for use as general purposes office copying machines. Therefore, prior art full color copiers have been limited to a specialized set of applications for which full color copies are needed on a regular basis, and the volume of such copies will justify the high cost of a full color copying machine. Virtually all such applications for prior art full color copying machines have been in environments which require the user to purchase or lease an additional monochrome copier to do more conventional black and white copying.
It is also known to those skilled in the art for any given toner material, or set of toner materials in the case of a color copier, a certain amount of heat per unit area must be transferred to the image receptor or paper carrying the toner in order to properly melt the plastic and fuse the image on the copy. The heat transfer is determined by a combination of the temperature on a rotating surface of the fuser which contacts the toner material, and the dwell time of the copy in the fuser. The dwell time is the time any given point on a particular copy is in contact with a heated surface within the fuser.
Conventional fusers are normally formed from a roller having a heated compressible surface and a compression roller which is urged against the heated compressible surface. The paper bearing the developed image of toner is passed between the heated roller and the compression roller. Since the compression roller deforms the compressible heated surface to some degree, there is a predetermined length along the path of travel of the paper for which the paper is in contact with the compressible surface of the heated roller. Therefore, dwell time may also be expressed as the product of the path length along the compressible surface of the heated roller for which the compression roller causes the copy to contact the compressible surface, times the linear rate of travel of the copy through the fuser mechanism.
As is known to those skilled in the art, the process of fusing a developed toner image onto the paper, or other image receptor, is often the rate limiting step in the copying process which limits the total copy per unit time output of a copying machine. As toners requiring greater amounts of heat for fusing become used, the only choices available to the designer are to increase the distance along the compressible surface of a heated roller for which the image receptor is in contact with the surface, to elevate the surface temperature of the heated roller in the fuser, or to slow down the rate of travel of the paper through the fuser.
Naturally, there is a practical upper limit to the temperature which can be maintained on the heated surface of a fuser in order to prevent the image receptor from catching fire or being scorched (in the case of paper) or melting (in the case of plastic transparencies), and also there is an upper temperature limit at which toner materials will have their color characteristics altered.
In conventional color copiers, all mechanical drive devices have been operated by a common prime mover in a manner which was mechanically synchronized. Therefore, the rate of travel of the image receptor bearing a developed full color image through the fuser mechanism has been substantially equal to the linear rate of movement of the image bearing surfaces within the machine. For example, the rate of travel of paper through the fuser in a conventional color copier has been equal to the rate of travel of the photoreceptor bearing the electrostatic image past the source of toner material. This has led to a trade-off in the design of such machines which requires that either the fuser temperature be elevated, thus increasing the power requirements of the machine, or that the entire operating speed of the machine be slowed down in order to provide sufficient dwell time for the copy within the fuser.
Prior art color electrophotographic print engines have had toner deposition modules which faced upward. In other words, the path from the toner and carrier reservoir to a decorator roll, which eventually carries the toner to the photoreceptor belt, was pointed upwardly, either vertically or in a direction with a predominant vertical component. It is believed that the conventional wisdom of those skilled in color electrophotography indicated that a plurality of color process toners could not be arranged in downwardly facing toner deposition modules in a practical machine.
One problem which exists in prior art color electrophotographic print engines is cross contamination of the color toners when a phenomenon known as carrier pull occurs. Carrier pull results from a condition of unusually high static charge on the photoreceptor. This normally occurs in response to a malfunction of one of the coronas which applies charge to the photoreceptor. The phenomenon of carrier pull is characterized by very strong electrostatic force between the highly charged portion of the photoreceptor and the toner particles on the portion of a decorator roller under the photoreceptor. These forces are so strong, that they pull large clumps of toner and its carrier particles off the decorator rollers onto the highly charged portion of the photoreceptor. As the photoreceptor continues to move, mechanical vibration can shake the particles loose where they will fall onto the decorator roller of a downstream toner module of another color. Note that this results, in large part, because the toner deposition modules are upwardly facing and the force of gravity tends to pull the excessive toner off the photoreceptor down onto the decorator rollers which become contaminated.
Also, when the phenomenon of carrier pull occurs, the magnetic particles which have been pulled off onto the photoreceptor tend to be attracted to the magnets of downstream decorator rolls. The combined influence of gravity and the magnets of the downstream decorator rolls which pull the carrier particles, and thus some of the color toner particles in the immediate vicinity, onto the other decorator rolls is another mechanism which causes contamination of other toner modules in the presence of carrier pull.
As is also known to those skilled in the art, the basic technology applied to xerographic copying has been applied more recently to devices known as laser printers. Conceptually, the print engine in a laser printer is substantially identical to the print engine used in xerographic copying machines. The fundamental difference between the two is the source of the image to be printed. In the case of a copier, an original is illuminated by a high-intensity light and the image from the illuminated original is focused onto the photoreceptor. In the case of a laser printer, control circuitry is used to turn a laser beam on and off as it sweeps a raster pattern over the surface of the photoreceptor to directly create the image to be reproduced. As the complexity of images to be created by the laser printer increases, and the pixel resolution increases, the computational power and memory requirements for circuitry associated with controlling the laser beam also increase. However, once the above-referenced electrostatic image is created on the photoreceptor, the principle of operation of a laser printer and a xerographic copier are identical. Color laser printing is accomplished in a manner analogous to color copying by computing a color value for each pixel and a relative intensity of the laser beam for each color component of the pixel color and creating three separate images, as described hereinabove.
The foregoing description is, to the best of the knowledge of the inventors of the present invention, an accurate description of the state of the art for color electrophotography prior to the invention of the present invention. From the foregoing it will be appreciated that the prior art has not produced a mechanism for a full color print engine which may be used in either a photocopying machine or a laser printer, which may be practically housed in an enclosure which is substantially the same size as conventional table-top convenience monochrome copiers. Furthermore, the prior art has not produced a full color print engine which will maintain the critical registration necessary to produce accurate full color copies by overlaying separate color component images without the use of relatively expensive finely machined mechanical parts. Additionally, the prior art has failed to produce a full color photocopying machine which will practically serve as a standard copier in most office environments by producing monochrome copies at either an adequate rate, or with acceptable copy quality for high volume monochrome copying. The latter limitation has been due to the substandard quality of process black as compared with conventional monochrome copiers using black toner. Also, the prior art has been unable to produce a practical full color photocopying machine for which the power consumption of the machine is controlled so that it may be operated from a conventional 120 volt 15 or 20 amp branch circuit.